While the present disclosure is described below primarily in connection with on-board electrical systems in vehicles, it is to be understood that the present disclosure may be used in any application in which electrical loads have to be switched.
Electronic semiconductor switching elements such as MOSFETs are increasingly being used as replacements for electromechanical relays in onboard electrical systems in vehicles. The advantages of semiconductor switching elements reside in a large number of switching cycles, lower power losses, shorter switching times, and noiseless switching. However, MOSFETs can break down, thereby losing their ability to switch off a current.
Current 40V MOSFETs with an on-state resistance of less than 0.7 ma (also referred to as drain-to-source on-state resistance) are constructed in accordance with so-called trench technology, which has replaced planar technology.
Contrary to planar MOSFETs, trench MOSFETs have vertical structures. They have shorter current paths and the cells are smaller. Thus, compared to planar MOSFETs, more cells can be accommodated with a higher density on the substrate. Therefore, several tens of thousands of cells can be parallel-connected on one die.
When a voltage is applied to a conventional MOSFET between a gate (i.e. a control input) and a source (i.e. a power output), a charge builds up on the gate and the NP channel in the particular cell becomes conductive. The gate is insulated from the P and N substrate. Thus, the gate functions in each case as a capacitor against the P and the N substrate.
In the event of a cell overload, the cell structure can be destroyed and the materials may intermix, resulting in a failure or breakdown. When this occurs, the barrier layer between the drain (i.e. power input) and the source (i.e. power output) is lost and the insulation of the gate terminal is destroyed. The MOSFET then becomes permanently conductive. As a result, the particular load can no longer be switched off.
U.S. Patent Application No. 2006/0250188 A1 discloses an amplifier system and test circuitry and a method for integrity testing of a power output field effect transistor (FET) of the amplifier system. Protective circuitry is described therein that makes overvoltage protection available for components of the amplifier system and switches off or isolates components during leakage measurements.
German Patent Document DE 10 2012 100 830 A1 describes a leakage current detection system. This system includes a pulse generator that feeds a pulse to a coupling capacitor; a voltage detector that detects a voltage at the coupling capacitor; and a leakage current determination unit that compares the voltage detected by the voltage detector to a threshold value and based on the result of the comparison determines a presence or absence of the electric current leakage of a direct current supply.
There are four basic failure mechanisms relating to semiconductor-based switching elements (referred to below simply as MOSFET):
1. Overvoltage causes a breakdown of the insulating layer of the gate and of the NP barrier layer. This process is analogous to a (puncture-like) overvoltage breakthrough in a capacitor.
2. Due to the excess current the powered cell is thermally overloaded and breaks down.
3. When an inductive load is switched off, the energy contained in the inductance is to be transformed in the switching element, i.e. the MOSFET, at least whenever the protective circuitry is inadequate or if there is no protective circuitry at all. The overvoltage induced by the current change results in surge releases of other charge carriers in the substrate (the so-called avalanche breakdown). In this way, the energy of the magnetic field surrounding the line becomes thermal energy in the MOSFET. The thermal energy can cause the destruction of the MOSFET celI(s).
4. The MOSFET is operated in the linear region since the gate is not adequately charged, with the gate-to-source voltage lying between 2V and 5V. The MOSFET cell now behaves in the manner of an adjustable resistor (i.e. transistor). Particularly in the case of trench MOSFETs, problems arise from the fact that the working point of the many parallel-connected cells is not identical. This means that some cells are more conductive than others. These cells transform correspondingly more energy loss, which can lead to the destruction of individual cells.
In view of the foregoing, there is a need for improved devices and methods for monitoring damage to semiconductor-based switching elements, such as MOSFETs. One of ordinary skill will understand from this disclosure that other uses for the presented embodiments are possible as well.